Various kinds of sound absorbers are used in a number of different disciplines for absorbing sound. Known sound absorbers include open-cell materials such as glass wool, rock wool, sponge, felt or urethane form; porous materials such as porous sintered board, metallic fiber board or foam metal board; open-cell chalk board; a combination of sheet material and nonwoven sound absorbers; or certain films.
Glass wool or felt has been used for absorbing sounds from engine compartments of automobiles. In this case, the sound absorbing coefficient may increase in proportion to the thickness of the sound absorber. The weight and cost of the sound absorber is larger as the thickness is increased.
A variety of sound absorbers are used for electric and electronic equipment. With the continuing emphasis on size reduction and cost reduction of such equipment, thin and flexible sound absorbers are desirable.
A variety of nonwoven sound absorbers have found use. But, the capability of some nonwoven fabrics is relatively low at low frequencies such as 3000 Hz or lower, and tends to be higher at higher frequencies. The sound absorption at low frequencies may be improved if the absorber is thicker. However, the sound absorbing capability at high frequencies may decrease.
It is known that the sound absorbing capability at low frequencies can sometimes be improved by using a combination of a porous material and a film. In this case, the sound absorber can be relatively thin, but a minimum amount of backing airspace (e.g. about 10 mm) may be necessary for best low frequency performance. In addition, the sound absorbing ability at high frequencies may be low.
It is desirable to provide sound absorbers or methods of absorbing sound which are capable of absorbing sounds in wide range of frequency, thin (even including a backing airspace), and have flexibility.